Constrainst for individual registers

[Ilya Lesokhin]

Hi,
I've used your suggestion of "Specifying Registers for Local
Variables" and found out that code it generates gives better results
than my method.

Unfortunately, a recent there on the subject:
http://gcc.gnu.org/ml/gcc/2012-01/msg00305.html

got me worried, so I would like to hear what you (or anyone familiar
with the subjet) know about the status of implementation of "Local
register Variables" in the AVR backend in gcc-4.5.3 and if it's better
in newer gcc's.

Specifically, I would like to know if you believe the following code
is safe (assuming _mul16x16_32 is implemented correctly):

DWord_t mul16x16_32(uint16_t x, uint16_t y)__attribute__((always_inline));	
DWord_t mul16x16_32(uint16_t x, uint16_t y)
{

	register uint16_t r24_25 asm ("r24") = x;
	register uint16_t r22_23 asm ("r22") = y;

	register uint16_t r30_31 asm ("r30");
	register uint16_t r20_21 asm ("r20");

	 asm (
		" call _mul16x16_32"  	"\n\t"
								: "=&r" (r20_21), "=&r" (r30_31)					: "r" (r24_25), "r"
(r22_23)					: "r0", "r1", "cc"
	);

	 DWord_t Result;
	 Result.High = r20_21;
	 Result.Low = r30_31;
	 return Result;
}

Thank,
Ilya.


On 10/24/11, Georg-Johann Lay <avr@gjlay.de> wrote:
>> Hi,
>> I'm doing some modifections to the avr backend to suit my needs.
>>
>> I would like to add new constrainst for individual registers to use in
>> inline assembly blocks and i was wondering what is the correct way of
>> doing this?
>
> Not on topic: You can have a look at GCC's "Specifying Registers for
> Local Variables" feature:
> http://gcc.gnu.org/onlinedocs/gcc/Local-Reg-Vars.html#Local-Reg-Vars
>
> For example, this enables you to interface non-ABI assembler functions
> with C code without patching the compiler.
>
> Moreover, introducing individual register classes will result in a zoo
> of constraints for 8-bit, 16-bit, 32-bit registers.
>
>> in gcc-4.5, i've added a new class for each register to reg_class,
>> REG_CLASS_NAMES and REG_CLASS_CONTENTS, made a need constrainst for
>> each register using its class and it worked as expected.
>>
>> i tried to do the same in gcc-4.7 and i got:
>> internal compiler error: in find_costs_and_classes, at ira-costs.c:1704.
>
> You will have to debug the compiler proper.
>
> Notice that hooks like HARD_REGNO_MODE_OK put additional restrictions on
> registers/mode combinations.
>
>> So i was wondering whether i did the wrong thing or just forgot to
>> update some target hook.
>>
>> Thanks,
>> Ilya.
>